The use of ion exchangers, both organic and inorganic, including crystalline molecular sieve zeolites, in order to remove certain metals from aqueous solutions is notoriously old in the art and the patent and technical literature contains many examples of such techniques. Although molecular sieves generally are effective for the removal of certain cations, nevertheless, when competing cations are present in the aqueous solution, a molecular sieve will function normally to the point at which the metal which is desirous of being removed effectively occupies some portion of the ionic sites in said zeolite. Thereafter, the zeolite must either be discarded or regenerated.
A very practical use for the above type of operation is in the home water softening industry wherein an ion exchanger of the organic or inorganic type is contacted with water until the calcium and magnesium ions which are inherently present in most mineral water replaces the ion originally associated with the ion exchanger, usually sodium. At this point, the ion exchanger has to be regenerated and this is usually accomplished by back-washing, or back-flushing, or otherwise contacting the ion exchanger with a solution of a different cation than that which was removed from the water, i.e., usually sodium in the form of sodium chloride. The sodium exchanges for the calcium/magnesium in the spent ion exchanger and the cycle is ready to start anew.
In evaluating the properties of a suitable ion exchanger, it is quite obvious that the environment in which it works to remove the unwanted metal or metals is of extreme importance and its susceptibility to competing ions is of paramount importance in obtaining a practical exchanger as opposed to one that is merely a scientific curiosity.
Thus, for example, in industrial processes wherein heavy metals are present in contaminated aqueous solutions, such heavy metals are not ordinarily present by themselves because the water contains other ions, particularly calcium and magnesium. Thus for an ion exchanger to be practical in the contact of industrial waste streams containing heavy metals, it is necessary that the ion exchanger be sufficiently selective towards heavy metals versus magnesium or calcium which compete for its ion exchange sites.
Another significant area where ion exchangers can be utilized is in the field of drinking water. The contamination of drinking water by toxic heavy metals, especially lead, has become a topic of great interest in both the scientific and popular press.
The Environmental Protection Agency (EPA) has stated that there is no threshold level of lead below which water is considered safe for human consumption. The EPA estimates that 138 million residents in the United States are potentially at risk from some degree of lead poisoning. Recognizing this problem, the National Sanitation Foundation (NSF) has established a Standard No. 53 which it recommends for adoption by the NSF Council of Public Health Consultants. Standard No. 53 sets a limit of no more than 20 parts per billion (20 ppb) of lead.
Sources of lead contamination include industrial waste as well as lead-bearing solders and other components found in the plumbing of most homes and water coolers.
There are various techniques utilized for the removal of lead, such as precipitation techniques. Such processes while effective in bulk removal are ineffective in reducing lead levels below about 50 ppb, an unacceptable level for human consumption. Other potential options for lead removal include purification of contaminated streams by exposure to synthetic ion exchange resins or various absorbents such as activated carbon. These systems typically suffer from low dynamic metal capacities, low lead selectivities and unacceptably slow rates of metal removal.
Cation exchange represents one potential avenue for the removal of many cationic metal species from aqueous systems. As indicated earlier, crystalline inorganic molecular sieves form the basis of hundreds of commercial ion exchange processes. Each individual molecular sieve demonstrates characteristic preferences of selectivities towards certain counterbalancing ions when exposed to mixed solutions and, thus, the separation or isolation of certain cations may be accomplished by the exposure to mixed cationic solutions to specifically tailored molecular sieves.
There are many types of devices used commercially for treating drinking water and they can generally be classified as under-the-tap, under-the-counter, whole-house treating systems. As the name implies, the under-the-tap device is one that is merely attached to the faucet or tap, the under-the-counter device is placed underneath the sink and the whole-house device is usually located in some central place, like the basement, wherein the entire water supply which enters the house is treated prior to it being distributed.
The three devices generally differ from each other in size. The under-the-tap device has a limited amount of space where the under-the-counter device has more space, and a whole-house treating system has the most space.
The under-the-tap and the under-the-counter devices usually contain a chamber which is occupied in part by activated carbon in order to remove organics from the drinking water and a limited amount of space for the inclusion of an ion exchanger. Quite obviously, the more effective the ion exchanger is, the less space it has to occupy and, correspondingly, more space can be occupied by activated carbon. Thus, the most stringent demands on an ion exchanger is for an under-the-faucet or under-the-tap application and, if an ion exchanger will function under these drastic conditions, then quite obviously it will also function in an under-the-counter device or larger devices which treat the entire water supply of the home.
In order to have the material for use as an ion exchanger in said under-the-faucet drinking water application, many problems must be overcome. The material must be able to remove lead from tap water to a level not greater than 20 ppb and to accomplish this in the presence of competing ions normally found in tap water. In order to be effective, the exchanger must have an extremely rapid rate of lead removal since the contact time in such an environment is extremely short. Additionally, because of the fact that most under-the-tap devices only allow a small amount of room or space which exchanger can occupy, this poses further restraint. Obviously, the rate of lead removal must be extremely high to meet the 20 ppb target. If more room were allowed for an ion exchanger, for example in an under-the-counter system, its rate though high could be less. However, as has been previously pointed out, it is still desirable to use an ion exchanger with an extremely high rate of lead removal even in an under-the-counter device since less of the material would be needed.
In addition to rapid lead removal, the exchanger must be able to retain the metal contaminants removed without causing what is referred to in the art as "avalanching." As is known in the art, when an exchanger removes a metal ion or ions from tap water, they gradually built up or accumulate on the ion exchanger so that the concentration in or on the ion exchanger is greater than the concentration of said ion or ions in the drinking water. If the ion exchanger suddenly releases such ions into the drinking water, a condition occurs when the drinking water has more contaminants than originally present in the untreated tap water. This is referred to as avalanching and, quite obviously, it must be avoided since it poses a significant health hazard.
It is to be understood that given enough time, all ion exchangers will avalanche or exhaust their capacity to absorb metal ions. However, since devices for treating drinking water, particularly under-the-tap and under-the-counter devices, generally contain activated carbon to remove organics which must be replaced at regular intervals, an ion exchanger must not avalanche or exhaust its capacity until significantly past the time when the activated carbon must be replaced. Thus, the life of the activated carbon becomes the limiting factor and not the ion exchanger.
It has now been discovered that certain amorphous titanium and tin silicate gels demonstrate remarkable rates of uptake for heavy metal species such as lead, cadmium, zinc, chromium and mercury which are an order of magnitude greater than that of prior art absorbents or ion exchangers under the conditions tested which include the presence of competing ions such as calcium and magnesium. The combination of extraordinary lead selectives, capacity and uptake rates, allows such materials to strip lead from aqueous streams with minimal contact time allowing direct end use in filters for water purification, be it under-the-counter or under-the-faucet, or whole-house devices.